This invention relates to a process of afterburning and purifying process exhaust gases which contain combustible constituents.
In various processes, e.g., in metallurgical processes carried out in rotary kilns, for instance, in the magnetizing roasting of iron ores and particularly in the direct reduction of iron ores to form sponge iron, exhaust gases which contain combustible constituents are generated. Depending on the process, the combustible constituents consist mainly of CO, H.sub.2, hydrocarbons and of solid carbon (carbon black) or dust, which is entrained by the gases, e.g., in the form of iron oxide. The contents of combustible constituents will be relatively high if coal has been used which has a relatively high content of volatile constituents. The combustible constituents must be afterburnt and the dusts and pollutants must be removed from the exhaust gas to avoid pollution.
It is known from U.S. Pat. No. 2,112,566 that the entire exhaust gas from a rotary kiln or a partial stream of such exhaust gas can be completely afterburnt by an addition of air in a single stage and the exhaust gas or a partial stream thereof can then be passed through a bed of solids on a traveling grate. By the complete afterburning in a single stage the temperature of the gas is increased to such a high level that the refractory lining of the afterburning chamber is subjected to strong thermal attack and that the melting or softening point of the fine dust is reached resulting in formation of deposits.
It is known from German Patent Specification No. 20 01 563 that the gas outlet end of a rotary kiln can be directly succeeded by an exhaust gas passage, which is cooled by a tubular wall and opens into the radiant heating chamber of a waste heat boiler. Immediately before the radiant heating section, oxygen-containing gases are supplied, which cause a complete afterburning to be performed. A partial stream of the completely afterburnt gas is withdrawn from the waste heat boiler, after it has been cooled to a temperature below the caking temperature of the entrained solids, and is used to heat the charge of a traveling grate. Whereas a formation of deposits can be avoided to a large extent by that practice, a directly succeeding waste heat boiler is required, which involves a considerable expenditure. Besides, laminar flow in the radiant heating section must be maintained to insure that softened solids cannot reach the wall and form deposits thereon.